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This patent disclosure includes Appendix A,
1. Field of the Invention
The present invention generally relates to semiconductor manufacturing equipment and more particularly to systems and methods for mapping semiconductor wafers.
2. Description of the Related Art
A semiconductor manufacturing equipment is used to process semiconductor wafers into electronic devices. Typically, in this equipment, the wafers are contained in a carrier. While in the carrier, the wafers are xe2x80x9cmappedxe2x80x9d to determine the number of wafers to be processed and the slot location of each wafer in the carrier. Wafer mapping enables the manufacturing equipment to return a processed wafer to the original slot location from which the wafer came. Returning the wafer into its original slot location in the carrier is important because equipment operators rely on slot locations to distinguish the wafers. Knowing the number of wafers contained in the carrier is also important because the manufacturing equipment needs to know when all the wafers in the carrier have been processed to alert an equipment operator or to move the out of a load lock.
Wafer mapping has been performed using laser beams. Typically, the carrier is slowly moved in a vertical direction to cross the path of a horizontally aimed laser beam. A computer keeps track of when and how many times the wafers break the beam. By knowing the distance between the slots of the carrier, the computer can determine the number of wafers and the location of each wafer in the carrier. Because each wafer in the carrier is moved to break the beam, using a laser beam to perform wafer mapping takes time. Further, detecting a cross-slotted wafer (i.e. a single wafer which occupies two slots) is difficult using a laser beam because of the limited area which a beam can cover.
The present invention provides a method and associated apparatus for mapping semiconductor wafers and wafer-like objects contained in a carrier or container. In one embodiment, an image of a carrier containing a wafer is acquired using, for example, a video camera. The image is digitized and stored in a computer as an array (i.e. row and column) of pixels, each pixel representing a point on the image. The intensity values of all pixels in a column of the image is then extracted. Because the presence of a wafer against a contrasting background generates an intensity variation in the pixels of the acquired image, the presence of the wafer can be determined by looking for variations in pixel intensity. A cross-slotted wafer is similarly detected by extracting the intensity of all pixels in two columns of the image. When a wafer is not cross-slotted, the pixel intensity variation corresponding to the wafer occurs on the same row of the image regardless of which column is being analyzed. The row location of a pixel intensity variation for a cross-slotted wafer, however, depends on the column of the image. This is because a cross-slotted wafer, which occupies two slots in the carrier, is at an angle with respect to the plane of a slot and thus intersects the columns of the image at different rows.
The invention only needs to acquire a single image of a carrier to determine the number and location of wafers in the carrier. This allows for a fast wafer mapping system with minimal moving parts. This is in marked contrast with techniques in the prior art where each wafer in the carrier is moved vertically to break a laser beam. The single acquired image can also be used to detect cross-slotted wafers, a task which is complicated and requires multiple sensors when the laser beam technique is used.